Genetic and environmental contributions to stability and change in children's internalizing and externalizing problems.

OBJECTIVE To estimate genetic, shared environmental, and nonshared environmental contributions to stability and change in internalizing and externalizing problems. METHOD Maternal Child Behavior Checklist ratings were obtained for 3,873 twin pairs at age 3 and 1,924 twin pairs at age 7. For 1,575 twin pairs, ratings were available at both ages. RESULTS For Internalizing/Externalizing ratings, genetic, shared, and nonshared environmental factors explained about 59/51%, 10/30%, and 31/19% of the variance at age 3, and 40/52%, 31/32%, and 29/16% of the variance at age 7. The phenotypic correlation of r = 0.38/0.54 between problems assessed at 3 and 7 years of age was explained for 66/55% by genetic factors, for 23/37% by shared environmental factors, and for 11/8% by nonshared environmental factors. The genetic, shared environmental, and nonshared environmental correlations between ages 3 and 7 were 0.51/0.57, 0.47/0.66, and 0.13/0.24, respectively. CONCLUSIONS Genetic and shared environmental factors were most important for the stability of Internalizing and Externalizing Problems between ages 3 and 7. Nonshared environmental factors were mainly age-specific. For Internalizing Problems, shared environment may become more important from early to middle childhood.